Process for lightening the color of polyol esters

ABSTRACT

The present invention relates to a process for lightening the color of polyol esters by reacting polyols with linear or branched aliphatic monocarboxylic acids having 3 to 20 carbon atoms, wherein the reaction product is worked up without using adsorbents and comprises a treatment with peroxidic compounds and an immediately subsequent steam treatment with subsequent drying.

CLAIM FOR PRIORITY

This application is based on German Application No. 10 2009 048 773.5,entitled “Verfahren zur Farbaufhellung von Polyolestern”, filed Oct. 8,2009, the priority of which is hereby claimed and the disclosure ofwhich is incorporated herein by reference.

TECHNICAL FIELD

The invention relates to a process for lightening the color of polyolesters formed from linear or branched aliphatic monocarboxylic acidshaving 3 to 20 carbon atoms by treating the polyol ester with peroxidiccompounds.

BACKGROUND OF INVENTION

Esters of polyhydric alcohols, also known as polyol esters, find avariety of uses on a large scale in industry, for example asplasticizers or lubricants. The selection of suitable starting materialsallows the physical properties, for example boiling point or viscosity,to be controlled, and the chemical properties, such as hydrolysisresistance or stability to oxidative degradation, to be taken intoaccount. Polyol esters can also be tailored to the solution of specificperformance problems. Detailed overviews of the use of polyol esters canbe found, for example, in Ullmann's Encyclopaedia of IndustrialChemistry, 5^(th) edition, 1985, VCH Verlagsgesellschaft, Vol. A1, pages305-319; 1990, Vol. A15, pages 438-440, or in Kirk 3rd Othmer,Encyclopaedia of Chemical Technology, edition, John Wiley & Sons, 1978,Vol. 1, pages 778-787; 1981, Vol. 14, pages 496-498.

The use of polyol esters as lubricants is of great industrialsignificance, and they are used particularly for those fields of use inwhich mineral oil-based lubricants meet the requirements made onlyincompletely. Polyol esters are used especially as turbine engine andinstrument oils. Polyol esters for lubricant applications are basedfrequently on 1,3-propanediol, 1,3-butanediol, 1,4-butanediol,1,2-hexanediol, 1,6-hexanediol, neopentyl glycol, trimethylolpropane,pentaerythritol, 2,2,4-trimethyl-pentane-1,3-diol, glycerol or3(4),8(9)-dihydroxy-methyltricyclo[5.2.1.0^(2,6)]decane, also known asTCD alcohol DM, as the alcohol component.

Polyol esters are also used to a considerable degree as plasticizers.Plasticizers find a variety of uses in plastics, coating materials,sealing materials and rubber articles. They interact physically withhigh molecular weight thermoplastic substances, without reactingchemically, preferably by virtue of their swelling and dissolutioncapacity. This forms a homogeneous system, the thermoplastic range ofwhich is shifted to lower temperatures compared to the originalpolymers, one result being that the mechanical properties thereof areoptimized, for example deformation capacity, elasticity and strength areincreased, and hardness is reduced.

In order to open up the widest possible fields of use to plasticizers,they must fulfil a series of criteria. They should ideally be odorless,colorless, and light-, cold- and heat-resistant. Moreover, it isexpected that they are insensitive to water, comparatively nonflammableand not very volatile, and are not harmful to health. Furthermore, theproduction of the plasticizers should be simple and, in order to meetecological requirements, avoid waste substances, such as by-productswhich cannot be utilized further and wastewaters comprising pollutants.

A specific class of polyol esters (they are referred to as G esters forshort) contains diols or ether diols as the alcohol component, forexample ethylene glycol, diethylene glycol, triethylene glycol,tetraethylene glycol, 1,2-propylene glycol or higher propylene glycols.They can be prepared in different ways. In addition to the reaction ofalcohol and acid, optionally in the presence of acidic catalysts,further processes are employed in practice to obtain G esters, includingthe reaction of diol with acid halide, the transesterification of acarboxylic ester with a diol, and the addition of ethylene oxide ontocarboxylic acids (ethoxylation). In industrial manufacture, only thedirect reaction of diol and carboxylic acid and the ethoxylation ofcarboxylic acids have become established as production processes,preference usually being given to the esterification of diol and acid.This is because this process can be performed with no particularcomplexity in conventional chemical apparatus, and it affords chemicallyhomogeneous products. Compared to this, ethoxylation requires extensiveand costly technical equipment. Ethylene oxide is a very reactivechemical substance. It can polymerize explosively and forms explosivemixtures with air within very wide mixing ranges. Ethylene oxideirritates the eyes and respiratory pathways, leads to chemical burns andto liver and kidney damage, and is carcinogenic. The handling thereoftherefore entails extensive safety measures. Moreover, scrupulouscleanliness of storage apparatus and reaction apparatus has to beensured, in order to rule out the formation of undesired impurities as aresult of side reactions of the ethylene oxide with extraneoussubstances. Finally, the reaction with ethylene oxide is not veryselective, since it leads to mixtures of compounds of different chainlength.

The direct esterification of alcohols with carboxylic acids is one ofthe basic operations in organic chemistry. In order to increase thereaction rate, the conversion is typically performed in the presence ofcatalysts. The use of one of the reactants in excess and/or the removalof the water formed in the course of the reaction ensures that theequilibrium is shifted in accordance with the law of mass action to theside of the reaction product, i.e. of the ester, which means that highyields are achieved.

Comprehensive information regarding the preparation of esters ofpolyhydric alcohols, also including esters of ethylene glycols and fattyacids, and regarding the properties of selected representatives of thesecompound classes can be found in Goldsmith, Polyhydric Alcohol Esters ofFatty Acids, Chem. Rev. 33, 257 ff. (1943). For example, esters ofdiethylene glycol, of triethylene glycol and of polyethylene glycols areprepared at temperatures of 130 to 230° C. over reaction times of 2.5 to8 hours. To remove the water of reaction, carbon dioxide is used.Suitable catalysts mentioned for the esterification of polyhydricalcohols are inorganic acids, acidic salts, organic sulphonic acids,acetyl chloride, metals or amphoteric metal oxides. The water ofreaction is removed with the aid of an entraining agent, for exampletoluene or xylene, or by introducing inert gases such as carbon dioxideor nitrogen.

The production and the properties of fatty acid esters of thepolyethylene glycols are discussed by Johnson (edit.), Fatty Acids inIndustry (1989) Chapter 9, Polyoxyethylene Esters of Fatty Acids, and aseries of preparative details are given. Higher diester concentrationsare achieved by the increase in the molar ratio of carboxylic acid toglycol. Suitable measures for removing the water of reaction areazeotropic distillation in the presence of a water-immiscible solvent,heating while passing through an inert gas, or performing the reactionunder reduced pressure in the presence of a desiccant. When the additionof catalysts is dispensed with, longer reaction times and higherreaction temperatures are required. Both reaction conditions can be mademilder by the use of catalysts. In addition to sulphuric acid, organicacids such as p-toluenesulphonic acid and cation exchangers of thepolystyrene type are the preferred catalysts. The use of metal powders,such as tin or iron, is also described. According to the teaching fromU.S. Pat. No. 2,628,249, color problems in the case of catalysis withsulphuric acid or sulphonic acids can be alleviated when working in thepresence of activated carbon.

One procedure in which esters of diethylene glycol and of triethyleneglycol and of caprylic acid are prepared without addition of catalyst isknown from U.S. Pat. No. 2,469,446. The esterification temperature is inthe range from 270 to 275° C. and the water of reaction is driven out bymeans of a carbon dioxide stream.

In the reaction regime in which the addition of a catalyst is dispensedwith, a molar excess of the particular carboxylic acid is generallyemployed, which, owing to its acidity, also acts as a catalyst.

For the removal of the water of reaction formed in the formation ofester from the polyol and the carboxylic acids, various processes areknown. For example, the water of reaction formed is distilled out of thereaction vessel together with the excess carboxylic acid and passed intoa downstream phase separator in which carboxylic acid and water separateaccording to their solubility properties. In some cases, the carboxylicacid used also forms an azeotrope with water under the reactionconditions, and is capable of removing the water of reaction as anentraining agent. Other methods employed include azeotropic distillationin the presence of an added water-immiscible solvent, heating of thereaction mixture while passing through an inert gas, the reaction of thepolyol and carboxylic acid starting materials under reduced pressure orin the presence of a desiccant. Especially the removal of water byazeotropic distillation has been found to be useful for theestablishment of the equilibrium in the preparation of polyol esters.According to the procedure known from DE 199 40 991 A1, thewater-immiscible solvent which acts as an entraining agent and must havea boiling point of less than 112° C. is added to the reaction mixtureonly on attainment of a temperature of at least 140° C.

In the industrial process, the mixture of water and carboxylic acidremoved is separated in a phase separator into the organic and aqueousphases, the aqueous phase is discharged and the carboxylic acid isrecycled back into the esterification reaction. For the workup of thecrude ester, for example, U.S. Pat. No. 5,324,853 A1 proposes removingexcess carboxylic acid by means of passage of nitrogen or steam, addingan adsorbent, neutralizing residual organic acid with a base, andfiltering off solids obtained. The residual amounts of acid present inthe filtrate are removed with the passage of steam or nitrogen withsimultaneous application of a reduced pressure and recycled back intothe esterification reaction. Solids obtained in the vacuum treatment areremoved in a final fine filtration. One task of the adsorbent added, forexample activated carbon, is to improve the color of the polyol ester.

According to the procedure known from U.S. Pat. No. 2,469,446 A1, thecrude ester obtained after removal of the water of reaction and ofexcess, unconverted starting materials, for example carboxylic acid, isfirst treated with an alkaline reagent, for example with an aqueoussodium carbonate or sodium hydroxide solution, in order to remove lastresidues of acidic constituents. After washing with water, and treatmentwith bleaching earth and activated carbon, the last traces of odoroussubstances can be removed by applying reduced pressure at elevatedtemperature. In some cases, the treatment with bleaching agents andactivated carbon has to be repeated more than once in order to producepolyol esters with satisfactory color properties.

Measures for improving the color of crude esters, such as oxidation, forexample with hydrogen peroxide or ozone, and the adsorption of activatedcarbon, are known from the general prior art, for example from H. Suter,Phthalsäureanhydrid and seine Verwendung [Phthalic anhydride and usethereof], Dr. Dietrich Steinkopf Verlag, Darmstadt 1972. To improve thecolor of ester compounds based on polyols, WO 94/18153 A1 proposes asubsequent treatment with an aqueous hydrogen peroxide solution. Theaction of ozone to lighten the color is discussed, for example, in DE 2729 627 A1.

Owing to the quality criteria described at the outset for polyol esters,the process steps in the esterification stage with removal of the waterof reaction and in the workup of the crude ester are very importantprocess features, since the adjustment of these process steps influencesthe sensory and optical properties of the end products to a significantdegree. More particularly, high demands are placed on the colorproperties, such as low color number and high color stability, of thepolyol esters. The structure of the starting materials, of thepolyhydric alcohols and of the acids, is, in contrast, crucial for themechanical and thermal properties of the polymer materials plasticizedwith the polyol esters and influences the hydrolysis and oxidationstability of lubricants.

The treatment with an adsorbent, for example activated carbon,high-surface area polysilicic acids, such as silica gels (silicaxerogels), kieselguhr, high-surface area aluminium oxides and aluminiumoxide hydrates, or mineral materials such as clays or carbonates, duringthe workup of the crude polyol ester to improve the color is aconventional process, but it requires additional filtration steps whichmean a considerable level of complexity in a process performedindustrially. Valuable product likewise remains adhering in the filterdevice and on the adsorbent, such that valuable product is lost in anadditional filtration step.

Treatment with hydrogen peroxide to improve the color can also be foundto be problematic since it can result in formation of organic peroxidesduring the treatment of the polyol esters. Traces of peroxides reducethe ester quality and the performance properties of the plasticizedpolymer products and of the lubricants produced on the basis of polyolesters. Peroxide traces also impair the storage performance of thepolyol esters, and an increase in the peroxide number is observed duringstorage in spite of exclusion of oxidizing agents such as air. To reducethe peroxide number, the prior art proposes an additional treatment witha reducing agent. Although this process is capable of reducing theperoxide number, such an operation means an additional working step inwhich the reducing agent has to be provided and removed again after usethereof.

SUMMARY OF INVENTION

It has now been found that, in the treatment of the crude polyol esterwith peroxidic compounds, light-colored products can be arrived atwithout using adsorbents when a treatment with peroxidic compounds and,immediately thereafter, without further intermediate steps, a treatmentwith steam are carried out and the polyol ester is finally dried, theconditions during the treatments, such as temperature to be employed,duration or pressure to be applied, being tailored to the particularpolyol ester.

Surprisingly, in this procedure, a light-colored polyol ester isobtained, which has an exceptionally low peroxide number which remainsstable and does not increase even over a prolonged storage period.

The invention therefore consists in a process for lightening the colorof polyol esters by reacting polyols with linear or branched aliphaticmonocarboxylic acids having 3 to 20 carbon atoms and then working up thereaction mixture without the use of adsorbents. The process ischaracterized in that removal of unconverted starting compounds isfollowed by treating the reaction product with peroxidic compounds,immediately thereafter performing a steam treatment without furtherintermediate steps and drying the remaining polyol ester.

The novel procedure is notable for great reliability not only inlaboratory and test operation, but in particular also in industrialplants. Even in continuous form, it is easy to perform and affordspolyol esters with high purity. The treatment of the crude ester withperoxidic compounds with immediately subsequent steam treatment andfurther drying leads to excellent color properties and remarkable colorstability of polyol esters, which additionally have only a low peroxidenumber. The peroxide number also remains stable at a low level over aprolonged storage time.

For the treatment of the crude ester obtained after removal ofunconverted starting compounds, suitable peroxidic compounds arehydrogen peroxide, organic percarboxylic acids such as peracetic acid orperpropionic acid, organic hydroperoxides such as cumene hydroperoxideor tert-butyl hydroperoxide, alkali metal or alkaline earth metalperborates, alkali metal or alkaline earth metal percarbonates, alkalimetal or alkaline earth metal peroxodisulphates or alkali metal oralkaline earth metal peroxophosphates.

Particularly suitable are aqueous hydrogen peroxide solutions, liquidorganic percarboxylic acids or organic hydroperoxides which can beremoved by distillation in a simple manner. The use of the peroxidicalkali metal or alkaline earth metal compounds in salt form, either insolid form or as an aqueous solution, is not ruled out but is restrictedto a few exceptional cases, since they and the reaction products thereofare present in solid form or precipitate in the course of workup of thecrude ester and have to be removed by an additional filtration step.

Especially suitable is hydrogen peroxide in the form of an aqueoussolution having a hydrogen peroxide content of more than 10% by weight,preferably of 30 to 50% by weight. Hydrogen peroxide solutions with alower active content are not recommended owing to the introduction oftoo high an amount of water which subsequently has to be removed again.In the case of excessive hydrogen peroxide concentrations, inconvenientand costly safety precautions have to be observed in the handling.

Further features and advantages will become apparent from the discussionwhich follows.

DETAILED DESCRIPTION

Generally, the peroxidic compound is added to the crude ester to betreated in such an amount that its active content in the overall mixtureis from 0.03 to 1.0% by weight, preferably from 0.08 to 0.3% by weight.In the case of excessively low active concentrations, the decolorizingpower is no longer sufficient to obtain light-colored polyol esters withsufficient quality. In the case of excessively high activeconcentrations, uncontrolled degradation reactions of the estercompounds have to be expected.

The treatment with peroxidic compounds is effected generally at elevatedtemperature, preferably at temperatures of 70 to 160° C., preferably 100to 120° C., though low temperatures, for example room temperature orlower, are not ruled out. The treatment time can be selected over a widerange. It should not be too short but not too long either, and can bedetermined by simple preliminary tests. In general, the treatment timeis 0.5 to 4 hours. In the case of shorter treatment times, no positiveinfluence on the color number is observed; in the case of excessivelylong treatment times, owing to the water present and the oxidizingagent, there is a risk of increased ester cleavage and uncontrolleddegradation of the polyol ester structure. Likewise, in the case ofexcessively long treatment times, reactor volume is occupiedunnecessarily.

The particular conditions of the treatment with the peroxidic compoundsshould be tailored to the particular polyol ester in order to achieveoptimal decolorization on the one hand, but as far as possible toprevent degradation reactions of the polyol ester on the other hand.Especially in the case of polyol esters based on ether diols, forexample triethylene glycol or tetraethylene glycol, increaseddegradation of the ether structure can set in when the conditions in thetreatment with the peroxidic compound, such as temperature, action timeand concentration, are not adjusted precisely to the particular polyolester.

After the oxidative treatment, the crude ester, without furtherintermediate steps, is subjected immediately thereafter to a treatmentwith steam, which can be effected, for example, in a simple form byintroducing steam into the crude product. One advantage of steamtreatment is that excess peroxidic compounds are destroyed in the coursethereof and residues of the starting compounds are removed with thesteam. Relatively large amounts of water still present are also drivenout by the steam treatment. At the same time, this measure improves thecolor number and the color stability of the crude ester.

The steam treatment is generally performed at standard pressure,although the employment of a slightly reduced pressure, appropriatelydown to 400 hPa, is not ruled out. The steam treatment is generallyperformed at temperatures of 100 to 250° C., preferably of 150 to 220°C. and especially of 170 to 200° C., and is also guided by the physicalproperties of the polyol esters to be prepared in each case.

In the process step of steam treatment, it is found to be appropriate toproceed in a very gentle manner during the heating period until theattainment of the working temperature, in order to heat the mixture ofcrude ester and added peroxidic compound to the required temperature forthe steam treatment.

The duration of the steam treatment can be determined by routine testsand it is generally performed over a period of 0.5 to 5 hours. Too longa steam treatment leads to an undesired increase in the color number ofthe polyol ester and should therefore be avoided. An increaseddegradation reaction of the polyol ester to acidic compounds is alsoobserved, the content of which is manifested in a rise in theneutralization number or acid number, for example determined accordingto DIN EN ISO 3682/ASTM D 1613. In the case of too short a treatmenttime, the destruction of excess peroxidic compound and traces of organicperoxides formed is incomplete, and the desired polyol ester still hastoo high an undesired peroxide number, expressed in milliequivalents ofoxygen per kilogram of product and determined according to ASTM E 298.Another observation in the case of too short a treatment time is only aminor advantageous effect on the color number of the polyol ester.

As in the case of the treatment with the peroxidic compound, theconditions in the immediately subsequent steam treatment, such astemperature, pressure and duration, also have to be adjusted preciselyto the particular polyol ester, in order to achieve an optimal result inrelation to the color number of the polyol ester and in order tominimize residual contents of starting compounds, water and of peroxidetraces as far as possible, and simultaneously to suppress degradationreactions. Especially in the case of polyol esters based on ether diols,for example triethylene glycol or tetraethylene glycol, the conditionsin the steam treatment have to be tailored exactly to the particularpolyol ester, in order to suppress the undesired degradation of theether chain.

Remarkably, the steam distillate which has been removed from the desiredpolyol ester and is obtained after condensation of the steam removedfrom the reaction section has a comparatively high peroxide number. Onthe industrial scale, the occurrence of large amounts of steam and steamdistillate with a high peroxide number can be found to be problematicfor safety reasons, since organic and possibly inorganic peroxides canbecome concentrated in the attached columns and distillate receivers. Ithas been found to be appropriate to contact the removed steam laden withwater and unconverted starting compounds, in which peroxides are alsopresent, with noble metals of groups 9 to 11 of the periodic table ofthe elements (according to IUPAC recommendation 1985), for example withpalladium or platinum. This measure can destroy the peroxide compoundspresent in the steam. The contacting is effected in gaseous form at thetemperature of the steam removed in the presence of the noble metals,by, for example, passing the steam over a commercial noble metalcatalyst in fixed bed form, which may either be supported orunsupported. For example, in a column section attached to the reactorsection, solid internals can be installed, which have a woven or porousstructure, for example a rectangular, honeycomb, round or othercustomary structure, to which the noble metals have been applied andthrough whose channels the gaseous and laden steam which has been passedthrough the crude ester and now removed passes. When the noble metal hasbeen applied to a support, suitable supports are those customary fornoble metal catalysts in industry, such as silicon dioxide, aluminiumoxide, activated carbon, titanium dioxide or zirconium dioxide in theirdifferent manifestations.

It is also possible to provide solid arrangements composed of noblemetals, for example fabrics, meshes, braids, wires, coils or sponges, inthe column section in order to destroy peroxide compounds driven outwith the steam.

It is also possible to treat the condensed liquid distillate removed, inwhich peroxides may be enriched, with noble metals of groups 9 to 11 ofthe periodic table of the elements to destroy peroxide compounds stillpresent, for example at autogenous temperature with commercial supportedor unsupported noble metal catalysts which may be used in fixed bed formor in suspension. It is also possible to contact a customary solidarrangement of noble metals, for example a fabric, a braid or wires, forexample a platinum mesh, with the liquid distillate removed.

The steam treatment is followed by the drying of the polyol ester, forexample by passing an inert gas through the product at elevatedtemperature. It is also possible simultaneously to apply a reducedpressure at elevated temperature and optionally to pass an inert gasthrough the product. Even without the action of an inert gas, it ispossible to work only at elevated temperature or only under reducedpressure. The particular drying conditions, such as temperature,pressure and time, can be determined by simple preliminary tests andshould be tailored to the particular polyol ester. In general, theworking temperatures are in the range from 80 to 250° C., preferably 100to 180° C., and the working pressures are from 0.2 to 500 hPa,preferably 1 to 200 hPa, and especially 1 to 20 hPa. After drying hasended, a light-colored polyol ester is obtained as the residue, withouta filtration step being required, in order to obtain on-spec product. Ina few exceptional cases, a filtration step may be required after thesteam treatment or after the drying when, for example, solid catalystresidues are not completely removed after the esterification reactionhas ended and after unconverted starting compounds have been removed,and hence before the workup of the reaction mixture.

In a particular configuration of the process according to the invention,the drying of the remaining polyol ester immediately follows the steamtreatment without further intermediate steps.

The reaction of polyols and aliphatic monocarboxylic acids can beperformed without use of a catalyst. This variant of the reaction hasthe advantage that addition of extraneous substances, which can lead toundesired contamination of the polyol ester, to the reaction mixture isavoided. However, it is then generally necessary to maintain higherreaction temperatures because only in this way is it ensured that thereaction proceeds with a sufficient, i.e. economically acceptable, rate.It should be noted in this context that the rise in the temperature canlead to thermal damage to the polyol ester. It is therefore not alwayspossible to avoid the use of a catalyst which facilitates the reactionand increases the reaction rate. Frequently, the catalyst may be anexcess of the aliphatic monocarboxylic acid, which is simultaneously areaction component of the polyol, such that the reaction proceedsautocatalytically. Otherwise, the customary esterification catalysts aresuitable for influencing the reaction rate, such as sulphuric acid,formic acid, polyphosphoric acid, methanesulphonic acid orp-toluenesulphonic acid, and equally combinations of such acids. It islikewise possible to use metallic catalysts, such as titanium-,zirconium- or tin-containing catalysts, for example the correspondingalkoxides or carboxylates. It is also possible to use catalyticallyactive compounds which are insoluble in the reaction system and solidunder reaction conditions, such as alkali metal or alkaline earth metalhydrogensulphates, for example sodium hydrogen-sulphate, although theuse of solid catalysts is restricted to a few exceptional cases, sincesolid catalysts have to be filtered out of the reaction mixture afterthe esterification has ended. In some cases, an additional finefiltration is also required during the workup of the crude polyol ester,in order to remove last residues of the solid catalyst. The amount ofthe catalyst used may extend over a wide range. It is possible to use0.001% by weight up to 5% by weight of catalyst, based on the reactionmixture. Since greater amounts of catalyst, however, give barely anyadvantages, the catalyst concentration is typically 0.001 to 1.0% andpreferably 0.01 to 0.5% by weight, based in each case on the reactionmixture. Appropriately, it may be decided by preliminary tests for eachindividual case whether to work without catalyst at higher temperatureor with catalyst at lower temperature.

The esterification can be undertaken with stoichiometric amounts ofpolyol and aliphatic monocarboxylic acid. Preference is given, however,to allowing the polyol to react with excess monocarboxylic acid withoutaddition of a catalyst, such that the excess monocarboxylic acid itselfacts as a catalyst. Excess monocarboxylic acid, which generally has alower boiling point than the polyol used, can also be removed from thecrude ester by distillation in a simple manner and a filtration step isdispensable owing to the avoidance of solid catalysts. The aliphaticmonocarboxylic acid is used in a 10 to 50% molar and preferably 20 to40% molar excess per mole of hydroxyl group to be esterified in thepolyol.

The water of reaction formed is distilled out of the reaction vessel inthe course of the reaction together with the excess monocarboxylic acidand passed into a downstream phase separator in which the monocarboxylicacid and water separate according to their solubility properties. Themonocarboxylic acid used may also form an azeotrope with water under thereaction conditions and be capable of removing the water of reaction asan entraining agent. The progress of the reaction can be monitored bythe water obtained. The water which separates out is removed from theprocess, while the monocarboxylic acid from the phase separator flowsback into the reaction vessel. The addition of a further organicsolvent, such as hexane, 1-hexene, cyclohexane, toluene, xylene orxylene isomer mixtures, which assumes the task of the azeotroping agent,is not ruled out, but restricted to a few exceptional cases. Theazeotroping agent can be added as early as at the start of theesterification reaction or on attainment of relatively hightemperatures. When the theoretical amount of water expected has beenobtained or the hydroxyl number, for example determined according to DIN53240, has fallen below a fixed value, the reaction is ended by allowingthe reaction mixture to cool.

The reaction between polyol and aliphatic monocarboxylic acid, dependingon the starting materials, sets in within the range from about 120 to180° C. and can be conducted to completion in different ways.

One configuration of the process according to the invention firstinvolves heating proceeding from room temperature to a temperature up toa maximum of 280° C., preferably up to 250° C., and, with thetemperature kept constant, lowering the pressure in stages proceedingfrom standard pressure, in order to facilitate the removal of the waterof reaction. The selection of the pressure stages, whether one, two ormore than two stages, and the pressure to be established at theparticular stage may be varied over a wide range and adjusted to theparticular conditions. For example, in a first stage, the pressure canbe lowered proceeding from standard pressure first down to 600 hPa, andthen the reaction can be conducted to completion at a pressure of 300hPa. These pressure figures are guide values which are appropriatelycomplied with.

In addition to the variation of the pressure, it is likewise alsopossible to alter the temperature proceeding from room temperature inone, two or more than two stages during the esterification reaction,such that, at constant pressure, the temperature is increased from stageto stage, typically up to a maximum temperature of 280° C. However, ithas been found to be appropriate to heat to a maximum of 280° C. withthe temperature rising from stage to stage, and also to lower thepressure from stage to stage. For example, the esterification reactioncan be conducted proceeding from room temperature in a first stage at atemperature up to 190° C. A reduced pressure down to 600 hPa is likewiseapplied, in order to accelerate the driving-out of the water ofreaction. On attainment of the temperature stage of 190° C., thepressure is lowered once again down to 300 hPa, and the esterificationreaction is conducted to completion at a temperature up to 250° C. Thesetemperature and pressure figures are guide values which areappropriately complied with. The temperature and pressure conditions tobe established at the particular stages, the number of stages and theparticular temperature increase or pressure reduction rate per unit timecan be varied over a wide range and adjusted in accordance with thephysical properties of the starting compounds and of the reactionproducts, the temperature and pressure conditions of the first stagebeing established proceeding from standard pressure and roomtemperature. It has been found to be particularly appropriate toincrease the temperature in two stages and to lower the pressure in twostages.

The lower limit of the pressure to be established depends on thephysical properties, such as boiling points and vapour pressures, of thestarting compounds and of the reaction products formed, and is alsodetermined by the plant apparatus. Proceeding from standard pressure, itis possible to work in stages within these limits, with pressuresdecreasing from stage to stage. The upper temperature limit, typically280° C., should be complied with in order to prevent the formation ofdecomposition products, which adversely affect color among otherproperties. The lower limit of the temperature stages is determined bythe reaction rate, which must still be sufficiently high to complete theesterification reaction within an acceptable time. Within these limits,it is possible to work in stages with temperatures rising from stage tostage.

The reaction mixture obtained after the reaction has ended comprises, aswell as the polyol ester as the desired reaction product, possiblyunconverted starting materials, especially aliphatic monocarboxylic acidstill in excess, if an acid excess has been employed in accordance withthe preferred configuration of the process according to the invention.For workup, excess and unconverted starting materials are distilled off,appropriately with application of a reduced pressure. In order to removeacidic catalysts, such as dissolved sulphuric acid or solid potassiumhydrogensulphate, if added in the esterification stage, and in order toremove last residues of acidic constituents, it is also possible toprovide a treatment with an alkaline reagent, for example with anaqueous sodium carbonate or sodium hydroxide solution, or, inexceptional cases, a filtration.

Thereafter, the crude ester freed of the unconverted starting compoundsand any catalyst present is worked up according to the inventive measurecomprising treatment with peroxidic compounds, immediately subsequentsteam treatment and final drying, dispensing with the use of customaryadsorbents, such as activated carbon, high-surface area polysilicicacids such as silica gels (silica gel xerogels), kieselguhr,high-surface area aluminium oxides and aluminium oxide hydrates, ormineral materials such as clays or carbonates, during the workup.Without the use of adsorbents, light-colored polyol esters with asufficiently low peroxide number are obtained, which also satisfy theremaining specifications, such as water content, residual acid contentand residual content of monoester. The purified polyol ester remains,during the drying, as a residue in the reaction vessel with outstandingquality, and an additional filtration step is generally not required andis restricted only to a few exceptional cases.

The polyhydric alcohols or polyols used as starting materials for theprocess according to the invention satisfy the general formula (I)

R(OH)_(n)  (I)

in which R is an aliphatic or cycloaliphatic hydrocarbon radical having2 to 20 and preferably 2 to 10 carbon atoms, and n is an integer of 2 to8, preferably 2, 3, 4, 5 or 6.

Suitable polyols are likewise compounds of the general formula (II)

H—(—O—[—CR¹R²—]_(m)—)_(o)—OH  (II)

in which R¹ and R² are each independently hydrogen, an alkyl radicalhaving 1 to 5 carbon atoms, preferably methyl, ethyl or propyl, or ahydroxyalkyl radical having 1 to 5 carbon atoms, preferably thehydroxy-methyl radical, m is an integer of 1 to 10, preferably 1 to 8and especially 1, 2, 3 or 4, o is an integer of 2 to 15, preferably 2 to8 and especially 2, 3, 4 or 5.

Suitable polyols which can be converted by the process according to theinvention to light-colored polyol esters are, for example,1,3-propanediol, 1,3-butane-diol, 1,4-butanediol, neopentyl glycol,2,2-dimethylol-butane, trimethylolethane, trimethylolpropane,ditrimethylolpropane, trimethylolbutane,2,2,4-trimethyl-pentane-1,3-diol, 1,2-hexanediol, 1,6-hexanediol,pentaerythritol or dipentaerythritol or3(4),8(9)-dihydroxymethyltricyclo[5.2.1.0^(2,6)]decane.

Useful further polyols include ethylene glycol and 1,2-propylene glycol,and the oligomers thereof, especially the ether diols di-, tri- andtetraethylene glycol or dipropylene glycol, tripropylene glycol ortetra-propylene glycol. Ethylene and propylene glycols are industriallyproduced chemicals. The base substance for preparation thereof isethylene oxide and propylene oxide, from which 1,2-ethylene glycol and1,2-propylene glycol are obtained by heating with water under pressure.Diethylene glycol is obtained by ethoxylation from ethylene glycol.Triethylene glycol is obtained, like tetraethylene glycol, as aby-product in the hydrolysis of ethylene oxide to prepare ethyleneglycol. Both compounds can also be synthesized by reacting ethyleneglycol with ethylene oxide. Dipropylene glycol, tripropylene glycol,tetrapropylene glycol and higher propoxylation products are obtainablefrom the multiple addition of propylene oxide onto 1,2-propylene glycol.

To obtain light-colored polyol esters by the process according to theinvention, linear or branched, aliphatic monocarboxylic acids having 3to 20 carbon atoms in the molecule are used. Even though preference isgiven to saturated acids in many cases, depending on the particularfield of use of the plasticizers or lubricants, it is also possible touse unsaturated carboxylic acids as a reaction component for estersynthesis. Examples of monocarboxylic acids as components of polyolesters are propionic acid, n-butyric acid, isobutyric acid, n-pentanoicacid, 2-methylbutyric acid, 3-methylbutyric acid, 2-methyl-pentanoicacid, n-hexanoic acid, 2-ethylbutyric acid, n-heptanoic acid,2-methylhexanoic acid, cyclohexane-carboxylic acid, 2-ethylhexanoicacid, n-nonanoic acid, 2-methyloctanoic acid, isononanoic acid,3,5,5-trimethylhexanoic acid, 2-propylheptanoic acid, 2-methylundecanoicacid, isoundecanecarboxylic acid, tricyclodecanecarboxylic acid andisotridecane-carboxylic acid. The novel process has been found to beparticularly useful for the preparation of polyol esters of monoethyleneglycol, or of the oligomeric ethylene glycols and of 1,2-propyleneglycol, or of the oligomeric propylene glycols with C₄- to C₁₃- or C₅-to C₁₀-monocarboxylic acids, and for preparation of polyol esters basedon 1,3-butanediol, neopentyl glycol, 2,2,4-trimethylpentane-1,3-diol,trimethylolpropane, ditrimethylolpropane, pentaerythritol or3(4),8(9)-dihydroxymethyltricyclo[5.2.1.0^(2,6)]decane.

The polyol esters of ethylene glycol and the oligomers thereof areoutstandingly suitable as plasticizers for all common high molecularweight thermoplastic substances. They have been found to be particularlyuseful as an additive to polyvinyl butyral which is used admixed withglycol esters as an intermediate layer for production of multilayer orcomposite glasses. They can likewise be used as coalescence agents orfilm-forming assistants in aqueous dispersions of polymers which findvarious uses as coating materials. The preparation process according tothe invention makes it possible to prepare, in a simple manner, withoutthe use of customary adsorbents, polyol esters with outstanding colorproperties which also satisfy further quality demands, such as low odouror a low acid number. The process according to the invention isparticularly suitable for preparing triethylene glycoldi-2-ethylhexanoate (3G8 Ester), tetraethylene glycol di-n-heptanoate(4G7 Ester), triethylene glycol di-2-ethylbutyrate (3G6 Ester),triethylene glycol di-n-heptanoate (3G7 Ester) or tetraethylene glycoldi-2-ethylhexanoate (4G8 Ester).

The process according to the invention can be performed continuously orbatchwise in the reaction apparatus typical for chemical technology.Useful apparatus has been found to be stirred tanks or reaction tubesequipped with a heating apparatus and an attached column section.

The process according to the invention is illustrated in detail in theexamples which follow, but it is not restricted to the embodimentdescribed.

WORKING EXAMPLES

For the tests for color lightening, crude triethylene glycoldi-2-ethylhexanoate with a color number of 131 Hazen units was used,which was obtained by esterification of triethylene glycol with a 2.3molar amount of 2-ethylhexanoic acid without catalyst and addition ofentraining agent. The content determined by gas chromatography (% byweight) of triethylene glycol di-2-ethylhexanoate was 97.7%, that oftriethylene glycol mono-2-ethylhexanoate 1.2%, and the remainder to 100%was 1.1%.

The workup of the crude triethylene glycol di-2-ethylhexanoate wasperformed with in each case 300 g of crude product in a heatable 1 litrefour-neck flask which was equipped with stirrer, internal thermometerand dropping funnel. After addition of the aqueous hydrogen peroxidesolution and stirring under the reaction conditions described below, forthe subsequent steam distillation, the dropping funnel was replaced by adistillation apparatus with a 1 litre receiver and the 1 litre four-neckflask was provided with an immersed tube for passage of steam. In thedistillation column was positioned a platinum mesh through which theperoxide-laden steam driven out was passed.

After performing the steam distillation under the conditions describedbelow, the supply of steam was stopped and a reduced pressure wasapplied over the distillation apparatus for final drying. The residueobtained was a light-colored, on-spec polyol ester without the use ofadsorbents.

Example 1

The crude triethylene glycol di-2-ethylhexanoate was treated with anaqueous hydrogen peroxide solution under the following conditions:

Concentration of the aqueous H₂O₂ 30% by weight solution Amounts ofH₂O₂, absolute based on the 0.10% by weight overall reaction mixtureReaction temperature 120° C. Reaction time 1 hour

The immediately subsequent steam distillation was performed using aplatinum mesh under the following conditions:

Working temperature of the steam 180° C. distillation Treatment time 1hour

Subsequently, the following drying conditions were established:

Pressure 10 hPa Drying temperature 140° C. Drying time 0.5 h

On completion of the workup, a light-colored polyol ester was obtainedwith the following contents determined by gas chromatography:

Triethylene glycol di-2-ethylhexanoate 97.9% by weight  contentTriethylene glycol mono-2- 0.9% by weight ethylhexanoate contentRemainder 1.2% by weightand the following indices:

Hazen color number (DIN ISO 6271) 29 Neutralization number (mg KOH/g,DIN EN 0.05 ISO 3682/ASTM D 1613) Water content (% by weight, DIN 517770.05 Part 1) Peroxide content (meq O/kg, ASTM E 298) 1.15

In the distillate of the steam distillation, a peroxide content of 3.0meq O/kg was found.

Example 2

Example 2 was carried out according to Example 1 with the sole exceptionthat the steam distillation was effected without the use of a platinummesh. The distillate obtained had a peroxide content of 13 meq O/kg. Theindices of the purified polyol ester corresponded to the valuesdisplayed according to Example 1.

Example 3

Preparation of NPG di-2-ethylhexanoate with subsequent treatment withhydrogen peroxide.

The esterification of neopentyl glycol with 2-ethyl-hexanoic acid wasperformed in a heatable 1 litre four-neck flask which was equipped withstirrer, internal thermometer and a water separator.

The flask was initially charged with 312.75 grams (3.00 mol) ofneopentyl glycol and 966.89 grams (6.70 mol) of 2-ethylhexanoic acid.While stirring and with application of a reduced pressure of 600 hPa,the mixture was heated to 200° C. and left to react under theseconditions for 2 hours. Subsequently, the pressure was reduced stepwiseto 500 hPa, and water of reaction formed was removed from the waterseparator. The progress of the reaction was monitored by continuouslyweighing the water discharged via the water separator, and by thevariation of the hydroxyl number. After a total of 8 hours of reactiontime, the reaction was ended at a residual hydroxyl number of 4.2 mgKOH/g (according to DIN 53240).

Subsequently, the excess 2-ethylhexanoic acid was distilled off over aperiod of 2 hours at a temperature of 190° C. and at a pressure of 95hPa, and additionally over 30 minutes at a temperature of 130° C. and apressure of 6 hPa. There followed the treatment with a 30% by weighthydrogen peroxide solution in an amount of 0.1% by weight of absolutehydrogen peroxide, based on the reaction mixture, at a temperature of120° C. over a period of one hour.

The subsequent steam distillation was performed at standard pressurewhile passing steam through at a temperature of 180° C. over a period ofone hour. Subsequently, the steam distillation was stopped and apressure of 10 hPa was applied over the distillation apparatus for finaldrying. The drying was performed at 140° C. over a period of 30 minutes.The residue obtained was light-colored neopentyl glycoldi-2-ethylhexanoate with the following indices:

Contents Determined by Gas Chromatography:

Neopentyl glycol di-2-ethylhexanoate 92.7% by weight  content Neopentylglycol mono-2-ethylhexanoate 6.2% by weight content Remainder 1.1% byweight

Indices:

Hazen color number (DIN ISO 6271) 49 Neutralization number (mg KOH/g,DIN EN 0.26 ISO 3682/ASTM D 1613) Water content (% by weight, DIN 517770.02 Part 1) Peroxide content (meq O/kg, ASTM E 298) 1.59

Example 4 Comparative Example

The esterification of neopentyl glycol with 2-ethyl-hexanoic acid andthe subsequent removal of the unconverted and excess 2-ethylhexanoicacid were performed according to Example 3. Without the treatment withhydrogen peroxide, the subsequent steam treatment was effected at 180°C. over a period of 30 minutes, and the subsequent drying at 120° C.over 15 minutes. The neopentyl glycol di-2-ethylhexanoate obtained inthe residue exhibited the following indices:

Contents Determined by Gas Chromatography:

Neopentyl glycol di-2-ethylhexanoate 92.9% by weight  content Neopentylglycol mono-2-ethylhexanoate 6.2% by weight content Remainder 0.9% byweight

Indices:

Hazen color number (DIN ISO 6271) 140 Neutralization number (mg KOH/g,DIN EN 0.17 ISO 3682/ASTM D 1613) Water content (% by weight, DIN 517770.02 Part 1)

The inventive measure of treating the crude esterification mixture withhydrogen peroxide after removing unconverted starting compounds, andimmediately thereafter performing a steam treatment without furtherintermediate steps, produces light-colored polyol esters with high colorstability without the use of adsorbents. In a further configuration ofthe process according to the invention, the steam driven out during thesteam treatment can be contacted with a platinum mesh. This measure cansignificantly deplete the peroxide content in the distillate removed,which avoids safety problems which would have to be managed in the caseof occurrence of amounts of distillate with a high peroxide content.

While the invention has been described in detail, modifications withinthe spirit and scope of the invention will be readily apparent to thoseof skill in the art. In view of the foregoing discussion, relevantknowledge in the art and references discussed above, the disclosures ofwhich are all incorporated herein by reference, further description isdeemed unnecessary. In addition, it should be understood that aspects ofthe invention and portions of various embodiments may be combined orinterchanged either in whole or in part. Furthermore, those of ordinaryskill in the art will appreciate that the foregoing description is byway of example only, and is not intended to limit the invention.

1. Process for lightening the color of polyol esters by reacting polyolswith linear or branched aliphatic monocarboxylic acids having 3 to 20carbon atoms and then working up the reaction mixture without the use ofadsorbents, characterized in that removal of unconverted startingcompounds is followed by treating the reaction product with peroxidiccompounds, immediately thereafter performing a steam treatment withoutfurther intermediate steps and drying the remaining polyol ester. 2.Process according to claim 1, characterized in that the active contentof peroxidic compound is 0.03 to 1.0% by weight, preferably 0.08 to 0.3%by weight, based on the overall mixture.
 3. Process according to claim1, characterized in that the peroxidic compound is selected fromhydrogen peroxide, organic percarboxylic acids, organic hydroperoxides,alkali metal or alkaline earth metal perborates, alkali metal oralkaline earth metal percarbonates, alkali metal or alkaline earth metalperoxodisulphates and alkali metal or alkaline earth metalperoxophosphates.
 4. Process according to claim 3, characterized in thathydrogen peroxide is used in the form of an aqueous solution having ahydrogen peroxide content of more than 10% by weight, preferably of 30to 50% by weight.
 5. Process according to claim 1, characterized in thatthe treatment with peroxidic compounds is effected at temperatures of 70to 160° C., preferably 100 to 120° C.
 6. Process according to claim 1,characterized in that the steam treatment is performed at a temperatureof 100 to 250° C., preferably of 150 to 220° C. and especially of 170 to200° C.
 7. Process according to claim 1, characterized in that the steamremoved in the steam treatment is contacted in gaseous form with noblemetals of groups 9 to 11 of the periodic table of the elements. 8.Process according to claim 1, characterized in that the steam removed inthe steam treatment is first condensed and the condensed liquiddistillate is contacted with noble metals of groups 9 to 11 of theperiodic table of the elements.
 9. Process according to claim 7,characterized in that the noble metals of groups 9 to 11 of the periodictable of the elements are in fixed bed form.
 10. Process according toclaim 9, characterized in that the noble metals of groups 9 to 11 of theperiodic table of the elements have been applied to a support. 11.Process according to claim 10, characterized in that the support used issilicon dioxide, aluminium oxide, activated carbon, titanium dioxide orzirconium dioxide.
 12. Process according to claim 9, characterized inthat the noble metals of groups 9 to 11 of the periodic table of theelements are arranged in the form of a fabric, mesh, braid, wire, coilor sponge.
 13. Process according to claim 7, characterized in that thenoble metals of groups 9 to 11 of the periodic table of the elementsused are palladium or platinum.
 14. Process according to claim 1,characterized in that the polyol ester is dried at temperatures of 80 to250° C., preferably 100 to 180° C., and at pressures of 0.2 to 500 hPa,preferably 1 to 200 hPa and especially of 1 to 20 hPa.
 15. Processaccording to claim 1, characterized in that the remaining polyol esteris dried immediately after the steam treatment without furtherintermediate steps.
 16. Process according to claim 1, characterized inthat the polyols used are compounds of the general formula (I)R(OH)_(n)  (I) in which R is an aliphatic or cycloaliphatic hydrocarbonradical having 2 to 20 and preferably 2 to 10 carbon atoms, and n is aninteger of 2 to 8, preferably 2, 3, 4, 5 or
 6. 17. Process according toclaim 1, characterized in that the polyols used are compounds of thegeneral formula (II)H—(—O—[—CR¹R²—]_(m)—)_(o)—  (II) in which R¹ and R² are eachindependently hydrogen, an alkyl radical having 1 to 5 carbon atoms,preferably methyl, ethyl or propyl, or a hydroxyalkyl radical having 1to 5 carbon atoms, preferably the hydroxymethyl radical, m is an integerof 1 to 10, preferably 1 to 8 and especially 1, 2, 3 or 4, o is aninteger of 2 to 15, preferably 2 to 8 and especially 2, 3, 4 or
 5. 18.Process according to claim 16, characterized in that the polyols usedare 1,2-propanediol, 1,3-propanediol, 1,3-butanediol, 1,4-butanediol,neopentyl glycol, 2,2-dimethylolbutane, trimethylolethane,trimethylolpropane, trimethylolbutane, 2,2,4-trimethylpentane-1,3-diol,1,2-hexanediol, 1,6-hexanediol, pentaerythritol, ethylene glycol or3(4),8(9)-dihydroxymethyltricyclo[5.2.1.0^(2,6)]decane.
 19. Processaccording to claim 17, characterized in that the polyols used areditrimethylolpropane, dipentaerythritol, diethylene glycol, triethyleneglycol, tetraethylene glycol, dipropylene glycol, tripropylene glycol ortetrapropylene glycol.
 20. Process according to claim 1, characterizedin that the aliphatic monocarboxylic acid converted is propionic acid,n-butyric acid, isobutyric acid, n-pentanoic acid, 2-methylbutyric acid,3-methylbutyric acid, 2-methylpentanoic acid, n-hexanoic acid,2-ethylbutyric acid, n-heptanoic acid, 2-methylhexanoic acid,2-ethylhexanoic acid, n-nonanoic acid, 2-methyloctanoic acid,isononanoic acid, 3,5,5-trimethylhexanoic acid or 2-propylheptanoicacid.
 21. Process according to claim 1 for preparing triethylene glycoldi-2-ethylhexanoate, tetraethylene glycol di-n-heptanoate, triethyleneglycol di-2-ethylbutyrate, triethylene glycol di-n-heptanoate ortetraethylene glycol di-2-ethylhexanoate.